1. Technical Field
This invention relates to bladed rotor assemblies, and especially to bladed rotor assemblies for gas turbine engines.
2. Background Information
Bladed rotor assemblies are well known in the art, such as for compressors and turbines of gas turbine engines. In such assemblies, each blade is often attached to the rotor disk by means of a root, integral with the radially innermost end of the blade. The root fits closely within a corresponding blade root slot extending generally axially through the disk rim, but at an angle to the true direction of the disk axis. The disk material disposed circumferentially between a pair of adjacent slots is often referred to as a disk lug. The blade root includes radially outwardly facing reaction surfaces that engage corresponding radially inwardly facing reaction surfaces of a blade root slot. During operation of the rotor, the blade loads are transferred into the disk and disk lugs through these engaged surfaces. Typically, a blade root extends from the front face to the rear face of the disk; and the engaged load reaction surfaces also extend from the front to the rear face of the disk (i.e. the full length of the slot). This is true of bladed disks having conventionally designed dovetail shaped roots and slots, as well as fir tree shaped roots and slots.
It is generally desired to keep stresses within the disk and within the blades as low as possible to extend part life. In gas turbine engines designed for flight, it is also desired to minimize the weight of parts, such as disks and blades, consistent with efficient operation, long life and safety. Lighter weight blades also generate lower centrifugal forces and thus may reduce stresses within the disk.
In accordance with the present invention, a bladed rotor disk assembly includes a plurality of circumferentially spaced apart blade root slots extending through the disk at an angle to the disk axial direction and having radially inwardly facing load reaction surfaces extending continuously over less than the full the length of the slot in contact with a corresponding radially outwardly facing load reaction surface of a blade root disposed within the slot.
By xe2x80x9cload reaction surfacexe2x80x9d, it is meant the surfaces of the blade root and blade root slot that, during operation of the rotor, contact or engage each other to transfer the loads from the blade into the disk. When in contact these surfaces form a xe2x80x9cload transfer interfacexe2x80x9d.
More specifically, the present invention eliminates what in the prior art would be portions of the load transfer interface adjacent the ends of the blade root slot, such that the loads over the remaining load transfer interface result in one or more of the following: a more symmetrical load distribution resulting in reduced torque loads on the disk lugs; reduced total loads on the disk lugs and blade roots; and, reduced maximum stress levels in the disk lugs and blade roots.
One reason these benefits may occur is because, with conventional root and slot designs, when the blade root load reaction surface along a side of a blade root extends the full length of the slot, the highest and most concentrated reaction loads on that side of the slot occur adjacent one end of the slot, while relatively lower and less concentrated (i.e. more uniform) reaction loads on that same side of the slot occur adjacent the other end of the slot. Therefore, at the low, more uniform reaction load end of the slot, the disk lug material is carrying a relatively small portion of the blade load per square inch of load transfer interface, while at the high reaction load end the disk lug material is carrying a much larger portion of the blade load per square inch of load transfer interface. By eliminating load transfer interface area at the low load end of each side of a slot, the reaction loads over the remaining load transfer interface on each side of the slot becomes more balanced, and results in lower maximum stress.
In one embodiment of the present invention a small area of each side of a slot adjacent an end of the slot and which faces what would normally be the low load portion of the root reaction surface is instead spaced from that low load portion such that there is a gap between the blade root and slot over that area. In all other respects, the blade and disk assembly may be considered conventional. The reaction loads over the now smaller load transfer interfaces on each side of the blade root are more balanced than without the gaps, and the maximum stress in the disk lugs is reduced.
In another embodiment of the present invention, end portions of the conventional blade root that normally transfer relatively low loads into the disk lugs are removed, providing the benefit of reduced blade weight in addition to more balanced reaction loads over the remaining length of a smaller load transfer interface on each side of the blade root. Total reaction loads, stresses and/or torque on the disk lugs may thereby be reduced. Reduced torque loads means less twisting of the blade lugs, with correspondingly less twisting of the blades.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.